In recent years, progress has been made in the field of high-voltage motor drive apparatus with the intent of increasing the output of three-phase synchronous motors (hereinafter, motors). Consequently, the problem of inverter loss has greatly spread. Phase inversion is desired in order to increase output and voltage. To this end, Patent Literature 1 discloses technology for decreasing inversion loss while increasing efficiency and realizing a high-output motor. The motor drive apparatus of Patent Literature 1 performs switching according to a motor turn count, so as to use one of inverter circuits provided as a series-connected pair and inverter circuits provided as a parallel-connected pair. Specifically, the series-connected inverter circuits are used when the motor turn count is less than a predetermined turn count, and the parallel-connected inverter circuits are used when the motor turn count exceeds the predetermined turn count.
The pair of series-connected inverter circuits split a source voltage such that each inverter circuit generates split voltage (½ of the source voltage) and supplies the split voltage to drive the motor. Accordingly, switching loss in switching elements making up the inverter circuits is decreased. In contrast, the parallel-connected pair of inverter circuits each generate power without splitting the source voltage to supply power to the motor. Accordingly, the motor is continually made to rotate despite high back-voltage occurring in motor coils when the motor turn count grows large.
According to the above, Cited Publication 1 continually causes the motor to rotate by having the inverter circuits drive the motor with high voltage when the motor has equaled or exceeded a predetermined turn count, yet decreases switching loss and achieves high efficiency and output while the turn count is low, which is the normal operating state of the motor.